TCP/IP (Transmission Control Protocol/Internet Protocol) is a network protocol used on LANs, WANs and the Internet. With the increasing amount of packet services on wireless systems, the use of TCP on a wireless system increases and in the near future it is believed that a considerable amount of the traffic on a wireless system is based on TCP/IP. Currently a major part of all Internet traffic is based on TCP/IP. Features of TCP/IP are described by Richard Stevens in TCP/IP Illustrated, vol. 1, Addison-Wesley, 1994, for example.
In TCP, data is split into what the protocol considers the optimum size chunks to transmit. The chunks are denominated segments and their size must not exceed a maximum constant value (Maximum Segment Size or MSS).
Whenever a segment is transmitted by a server it has to reach a client which acknowledges the transmitted segment. The elapsed time when the server transmits the segment until it receives the corresponding acknowledgment is commonly known as Round Trip Time (RTT). This is shown in FIG. 6 for a segment or packet S1 transmitted from the server to the client and an acknowledgment ACK S1 transmitted from the client to the server. The TCP server holds a variable used to calculate an estimation of a maximum allowed RTT. This variable is called RTO. Moreover, the server has a timer that counts the elapsed time since the segment was transmitted. If the corresponding acknowledgment does not arrive at the server before the timer reaches the value of the RTO estimator, the server considers that congestion occurred in the network and starts congestion avoidance actions.
TCP tries to adapt the transmission rate to the load and capacity of the links of the network. This is done by several mechanisms, like slow start, retransmission timeout, fast retransmission, etc. Fast retransmission and retransmission timeout cause retransmission of TCP/IP packets, when they are lost or delayed more than a dynamic timer RTO.
The flow control in TCP is accomplished by means of two windows: the congestion window (cwnd) and the advertised window (awnd). For the problem treated here, it will be assumed that the congestion window is the minimum of both windows, and therefore cwnd will characterize the maximum amount of outstanding data that the server may transmit. The congestion window is controlled by slow start and congestion avoidance algorithms that try to optimize the amount of data injected to the load situation of the Internet.
The slow start algorithm is executed in the beginning of a connection (just after connection set-up) or after the occurrence of a timer expiration (time out).
In FIG. 1 congestion window evolution in slow start and congestion avoidance is shown. During slow start, the congestion window is initialized to MSS bytes at the connection establishment (some implementations initialize it to two or four MSSs). After connection set-up the transmitting end passes to execute this algorithm. Each time a TCP acknowledgment is received at the server, the slow start increases the congestion window by MSS. So, when the first acknowledgment is received, the congestion window is increased from MSS to 2*MSS and two segments can be sent, and so on. FIG. 1 shows the exponential increase of the congestion window during slow start. When the cwnd exceeds the slow start threshold (e.g. initially set to 65535 bytes), the TCP transmitting entity (server) passes to execute the congestion avoidance algorithm. As it is illustrated in FIG. 1, when the congestion window reaches the slow start threshold it does not increase exponentially anymore but linearly.
Another different feature of TCP is denominated delayed acknowledgments. A host receiving a stream of TCP data segments can increase efficiency in both the Internet and the hosts by sending fewer than one ACK (acknowledgment) segment per data segment received. According to the Network Working Group, RFC 1122, this is known as a “delayed ACK”. As expressed in this document, TCP should implement a delayed acknowledgment, but an acknowledgment should not be excessively delayed; in particular, the delay must be less than 0.5 seconds, and in a stream of full-sized segments there should be an acknowledgment for at least every second segment.
The delayed acknowledgment procedure has the obvious benefit that the uplink traffic in the RAN and in the fixed network decreases. But, on the other hand, it produces a slower increase of the congestion window since it decreases the arrival pace of acknowledgments at the server. This is not desired during slow start because it enlarges the required time for the congestion window to reach its optimum value. This larger time required by the congestion window to reach its optimum value is perceived by the final user as a delay, resulting in a worse end throughput. The effect of this delay is more pronounced in the download of small files (such as small web pages) than in the download of very large files (such as ftp).
Disabling the delayed acknowledgment procedure at the client during slow start would result in faster increase of the congestion window during slow start, and thus would increase the end throughput. However, if acknowledgments increase the traffic too fast, this might cause congestion in some part of the RAN (Radio Access Network), CN (Core Network) or the Internet.
There are TCP implementations that do not use the delayed acknowledgments in the beginning of the downloading. For example, an implementation might acknowledge 4 first packets immediately and use delayed acknowledgment thereafter.
However, those limits are fixed and they do not take into account, for example, that the allocated bit rate for the user might not be constant in a radio access network e.g. during downloading.